1. Field
This disclosure relates generally to semiconductor processing, and more particularly, to improving reliability of isolated vias.
2. Related Art
Integrated circuits are formed with metal layers stacked on top of one another and dielectric layers between the metal layers to insulate the metal layers from each other. Normally, each metal layer has an electrical contact to at least one other metal layer. Electrical contact can be formed by etching a hole (i.e., a via) in the interlayer dielectric that separates the metal layers, and filling the resulting via with a metal to create an interconnect. A “via” normally refers to any recessed feature such as a hole, line or other similar feature formed within a dielectric layer that, when filled with a conductive material, provides an electrical connection through the dielectric layer to a conductive layer underlying the dielectric layer.
With the number of transistors that are now present on integrated circuits, the number of vias can exceed a billion and there can be ten or more different conductive layers. Even if each via is highly reliable, there are so many vias that it is likely for there to be at least one via failure. Low-k BEOL (Back-End of Line) interlayer dielectrics commonly used in advanced technology integrated circuit manufacturing can have trapped moisture and hydroxyl ions. These trapped water species pose a risk of oxidizing via barrier material if not sufficiently outgassed. Vias with oxidized tantalum barriers exhibit excessive via resistance that has been shown to cause timing delays in semiconductor devices.
A barrier material is used to contain the migration of a copper used for a metal layer through the insulating material. Barrier materials typically used today are a combination of tantalum and tantalum nitride, or just tantalum. Tantalum nitride has good adhesion properties to the oxide dieletric. However, other materials can be used. One problem which is specifically worse for tantalum is that tantalum oxidizes to form tantalum pentoxide and expands to a volume which is several times larger than just the tantalum. Also, the tantalum pentoxide is an insulator and has very high resistance.
Accordingly, it is desirable to provide a technique for improving the reliability of vias and uniformity of via resistance.